Self-processing photographic film units are now available which are completely self-contained and are adapted to be employed in a camera in which the film unit is exposed and then processed by being moved between a pair of pressure-applying members. The components of the film unit are assembled to form an integral structure and the integrity of this structure is maintained during exposure, processing and viewing, thereby making it unnecessary to store, handle and/or move separately, individual elements of the film unit, and minimizing the complexity of the structure required to contain and manipulate the film unit to effect exposure and processing thereof. Such a film unit structure is attractive, includes a minimum of simple and easily assembled components, is of a minimum size in relation to image size and generally includes substantially no excess materials.
The film units generally comprise two separate, flexible sheetlike elements including a first or image-recording sheet including a layer containing a photosensitive image-recording material and a second sheet for aiding in the distribution of a processing fluid as a layer of predetermined thickness in contact with an exposed area of the photosensitive material. A rupturable container filled with the processing fluid is disposed along one edge of the film unit, in a manner well known in the art. After the processing fluid has been spread from the leading end of the film unit between the sheets toward the trailing end therof. excess processing fluid is trapped and retained within the film unit at the trailing end thereof.
During spreading, the processing fluid is advanced between the sheet-like elements, or sheets, as a mass located immediately ahead of the pressure-applying members and extending from side-to-side of the film unit. Even if the film unit is assembled such that substantially all air is excluded from between the sheets by pressing them into face-to-face contact throughout substantially the entire area of the facing surfaces, it is difficult to exclude all air from the film unit during manufacture and, unless the binding element is adhered to the sheets so as to substantially prevent the entry of air, air may enter the film unit after manufacture during storage. It has been found that air (or any gas) located between the sheets during spreading of the processing fluid should be advanced ahead of the mass of fluid and should be permitted to escape from between the sheets at the trailing end of the film unit. Otherwise, the air or gas can be entrained as air bubbles in the rapidly advancing mass of fluid, with the bubbles forming voids or discontinuities in the layer of fluid. Moreover, it has been found that air pushed ahead of the mass of fluid can, if not permitted to escape, be compressed at the trailing end of the film unit during spreading, only to expand back into the image area after spreading is complete. This, too, forms voids or discontinuities in the fluid which are manifested in the image as discolored spots or regions.
Thus, it is necessary to provide for venting or otherwise expelling the air from between the sheets without inhibiting the spreading of the processing fluid, while at the same time preventing the processing fluid from escaping from between the sheets. It is known that escape of the processing fluid from between the sheets can be effectively prevented by the provision of trapping spaces and a liquid-tight retaining element secured to the outer borders of the sheets. Examples of trap structures and retaining elements are disclosed in U.S. Pat. Nos. 2,500,422, 2,627,460, 2,686,716, 3,589,904, 3,607,285, 3,615,540 and 3,619,193.
It is also known that a film unit can be vented in such a way as not to inhibit the release or escape of air, while at the same time minimizing the escape of processing fluid from the film unit. A number of expedients are known for releasing air from the trailing end of the film unit. For example, the trap cover may be perforated or slit in the trap region, as shown in U.S. Pat. Nos. 2,627,460, 3,615,540 and 3,619,193. The perforations or slits are preferably in the form of pinholes or very fine slits that are almost invisible, but which will readily pass air while resisting the passage of the viscous processing fluid. In other arrangements narrow regions of the binding element may be left unbonded to thereby provide for the escape of air, as shown in U.S. Pat. No. 3,589,904. In still other arrangements, the binding element or at least the trailing end portion thereof may be formed of a porous material which has pores of a size through which air may pass with a minimum of resistance and yet effectively preclude passage of the viscous processing liquid. Porous materials offering little resistance to the passage of air and essentially impermeable to viscous liquids include porous polymeric film and fibrous material commonly used as filters. Such materials may be coated with a water-proof agent to increase their impermeability to aqueous liquids without reducing their gas permeability. See U.S. Pat. No. 2,627,460.
However, it has been found with each of the above venting arrangements that prevention of the escape of the processing liquid is not completely assured. While many of those arrangements do generally preclude the escape of processing liquid during the spreading thereof, it is still possible for the user to subsequently, purposely or inadvertently, manipulate the trap portion of the film unit to such an extent that the processing liquid is forced out of the vent holes regardless of their size.